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What Are Optical Flats

by Naveen Agarwal
What Are Optical Flats

If you’ve ever been curious about optical flats and what they get used for, this article will get you up to speed. In fact, over the next three years, the market could grow by over $95,000,000. Keep on reading to learn what optical flats are for and how they get used.

What Are Optical Flats?

Optical flats are pieces of glass that get polished to make them as flat as possible. The measurement is usually 25 nanometers or less. They get made from fused quartz.

Optical flats use a monochromatic light source. That provides a single wavelength of light or color. The optical flat gets placed on top of the surface that gets tested.

When looking at the flat, the contour of the surface gets shown by white and dark bands. That reflects how flat the surface may be. More information is available on the capabilities of optical flats.

What Are Optical Flats Uses?

Optical flats get used for measuring the flatness of an object’s surface. The glass gets placed on top of the piece to get tested. When light waves combine, the ones reflected from the test piece have moved twice the distance of the gap.

If light gets shown on an optical flat that sits on top of an object’s surface, some light waves will reflect off the bottom of the flat. Other light waves will move to the surface of the piece tested, before showing reflection.

If the distance the reflection moves is a multiple of the wavelength, the waves line up when combined. That shows the troughs and crests of one will match the other wave. That constructive interference means the waves will reinforce each other.

When the distance the reflection traveled is a multiple and a half of the wavelength, that causes destructive interference. The crest of one wave will match up to the trough of the other one, which makes the waves 180 degrees out of phase. It will show a dark spot, which means they cancel each other out.

If the combination of the waves is in between constructive and destructive, the spot has intermediate intensity. White and dark spots make bands connect equal points of the gap between the flat and object tested. The theory is that interference fringes show the object is not flat.

There will always be a small gap because of air wedges that form between the flat and the surface of the test object. The farther across the wedge you go, the gap increases. The fringes made will be straight, parallel, and the same distance from each, other.

Types of Optical Flats

How many types of optical flats are there? There are two types, A and B. Type A only has one flat surface, where type B has two.

The testing side of type A has an arrowhead on the cylindrical surface that points to it. It measures other flats and slips. That includes the flatness of working faces.

The diameter and grade are vital for type A. It gets designated by these two characteristics.

Type B has two flat surfaces. They usually measure surfaces of micrometers and anvils. These get used for testing the parallelism of working faces.

Thickness and grade are the two factors for type B flats. They get designated by them.

Both types of optical flats get stored in wooden boxes lined with velvet to avoid damage. Dust and moisture can change the quality. The edges of these flats are both beveled to forty-five degrees.

Testing for Flatness

The flatness for both types A and get tested by comparing it with a master flat. The master has a known flatness. Sometimes the testing gets done by comparing the piece with three other flats that have unknown flatness rates.

Testing involves looking for interference fringes using a monochromatic light source. The rays could be parallel or diffused.

If the master’s flatness is known, it gets placed over the working surface of the one getting tested. If the flat getting observed has a flat surface, the fringes seen will be straight lines. The light and dark bands will be parallel to each other.

The deviation degree of the optical flat tested from the master flat gets shown by fringe deviation. They can go in any direction, which shows concavity or convexity.

These fringes are so small they get observed using a microscope. They get measured in width by utilizing cross wires.

If the master with the known flatness cannot get used, testing gets done by comparing two other flats of unknown flatness. The three get observed in pairs for interference fringes. The deviations for fringe straightness get monitored for all three cases.

Quality Testing of the Flat

Precision makes a difference when testing optical flats. You want to make sure they are clean and dirt-free before testing. You don’t want any fingerprints or oils on the glass.

The flat piece needs to get swabbed with alcohol before and after each test. During swabbing, flats need to get examined for any scratches. They need to get handled with care because they are fragile and easy to damage.

When testing, the optical flat should get placed on top of the object. Never slide the optical flat on or off another part because scratching can become a problem. If the object or optical flat gets scratched, the test measurements may not be accurate.

Practical Application

Whatever type of optical flats you use, there are practical applications for them. Any kind of metal ring, optical mirrors, and windows may get tested with optical flats.

Testing is vital to know the thickness and flatness of an object. Reflections from flat surfaces get used with anything from your bathroom mirror to telescopes. Check out our other articles for even more great information!

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